Bermuda is an isolated subtropical coral
reef ecosystem in the North Atlantic. Shallow water carbonates
cover an atoll-like volcanic seamount. The islands are emergent
aeolianite limestone dunes that lie along the southeastern margin
of the seamount. Extensive zones of coral reefs, dominated by
massive corals (Diploria spp., Montastraea
spp. and Porites astreoides), surround
the central North Lagoon. Within the lagoon there is a complex of
shallow patch reefs that support a higher diversity of corals,
dominated by branching species (Oculina
spp., Madracis spp.) and the hydrozoan Millepora
alcicornis. Seagrass beds are distributed throughout
the patch reefs, the inshore basins, and the shoreward margin of
the outer rim reef. Three species of seagrasses, Thalassia
testudinum, Syringodium filiforme,
and Halodule bermudensis, are often
intermixed or form monospecific beds. Mangrove forests have been
reduced by foreshore development to small pockets and fringing
communities, except at Hungry Bay. Rhizophora mangal
and Avicennia germinans are the only
species present. Seawater temperature fluctuates seasonally
within the lagoon (14-31°C) and on the outer reef (18-29°C),
which is moderated by the surrounding Sargasso Sea. Salinity
remains close to oceanic values (36.5) due to low run-off
from the porous limestone islands. Despite a high population
density, human impacts are limited to over-fishing (now
controlled through legislation), nutrient and trace metal loading
of the inshore basins, and ship groundings.

Bermuda is a unique island ecosystem located at
a high latitude (32°N, 65°W) in the North Atlantic. The warming
influence of the nearby Gulf Stream moderates the air and water
temperature, allowing the development of subtropical marine and
littoral communities (coral reefs, seagrass beds, mangrove
forests). The islands of Bermuda are emergent aeolian dunes,
formed during Pleistocene interglacial periods, located along the
southeastern edge of the Bermuda Pedestal (Garret and
Scoffin, 1977;Mackenzie and Vacher, 1975;Morris et al.,
1977).

The volcanic pedestal is covered by Pleistocene and Holocene
carbonates, 15 to 100 m thick, and the upper surface is about 665
km2 in
size of which only 50 km2 is covered by the islands. The bulk of the upper
surface of the pedestal is a shallow lagoonal system (<20 m
deep) with extensive shallow reefs, seagrass beds, and deeper
muddy basins, referred to as the North Lagoon (Fig. 1).
The islands form protected nearshore areas with further seagrass
development and isolated pockets of mangroves. Extensive reef
zones are developed at the seaward margin and flanks of the
pedestal (Fig. 1). The outer reefs and lagoon areas, <20 m deep, are
referred to collectively as the Reef Platform (Logan, 1988).

The marine biota is Caribbean in origin, with
reduced species diversity and relatively little endemism (Sterrer, 1986). The cool winter water temperatures (14-18°C) are
believed to be the limiting factor for the survival of some
Caribbean species, although the degree of isolation of Bermuda
(about 1200 km from The Bahamas and Florida) may limit larval
dispersal from the Caribbean (Logan, 1988).

The islands were colonized in the early 17th century and extensive
development has taken place, primarily since 1940. At present,
the population is about 60,000, distributed throughout the
islands. The economy is based principally on tourism,
international business, and finance (Hayward et al., 1981). The main settlements are the city of Hamilton in the
center of the island and the town of St. George at the eastern
end. Both urban areas have centralized sewage systems that
discharge directly into the marine environment with little
treatment; the Hamilton outfall is located on the South Shore and
the St. George system empties into the North Lagoon.

Human
impacts on the marine environment have been relatively limited.
Fishing pressure on fish, conchs, and lobsters on the reefs has
increased throughout the 20th century and stocks have declined. A
reasonably comprehensive management plan was introduced in 1990,
including restricted areas and endangered species protection, to
stabilize harvesting efforts (Butler et al., 1993). The inshore coastal lagoons receive nutrient inputs
derived from agriculture and sewage via both terrestrial runoff
and groundwater seepage, and also trace metal contaminants (Bodungen et al.,
1982;Jickells et al., 1986).
Direct sewage inputs appear to have limited impact due to good
flushing at the outfall sites. Foreshore development has been
extensive, with the creation of commercial and naval port
facilities. Also, shipping channels have been dredged and blasted
through the North Lagoon for access to the capital city of
Hamilton at the center of the island and to the naval bases at
the western end of the island. The creation of an airfield by
dredging and filling a large area in Castle Harbour eliminated
coastal mangroves, caused coral mortality, and altered the
ecology of the lagoon (Dodge and Vaisnys, 1977;Dryer and Logan,
1978).

Bermudas climate and the oceanographic
conditions on the Reef Platform and surrounding Sargasso Sea have
been studied intensively for decades (Anon., 1984;Morris et al., 1977). A comprehensive summary of many types of data
pertaining to the Bermuda marine environment is given by Morris et al. (1977).

The high latitude of Bermuda (32°N) makes it
susceptible to strong low-pressure systems from North America,
bringing an average of 4-6 strong gales per winter (Anon., 1984; Garret and Scoffin, 1977).
High seas (5-20 m) are generated over a long fetch and impinge on
the outer reefs, qualifying them as high energy zones. The
lagoonal reefs experience 1-2 m waves during gales. Hurricanes
and tropical storms hit or pass by every 2-5 years and also
produce extreme wave conditions on the outer reefs.

Air temperature fluctuates strongly with the
seasons, dropping to 8°C with the passage of strong winter lows,
while summer highs range to 35°C (Table 1; Anon., 1984). Spring and summer air flow is generally from the SE
to SSW and usually less than 15 knots. ENE-N winds are common in
the autumn; winter air flow is from SW-W-NW, often >15 knots,
and frequently gale force (35 knots). Rainfall averages 147 cm
per year and is evenly distributed throughout the year. Cloud
cover varies seasonally, with more clear days in summer (35-40%)
than winter (15-20%). Solar insolation ranges from a July average
of 640 g cal1 cm2 d1 to a December average of 240 g cal1 cm2 d1.

The North Lagoon and outer reef zones differ
slightly in their oceanographic conditions (Boden and Kampa,
1953). The water temperatures on the
outer reefs range from 18 to 28°C, moderated by the passage of
Gulf Stream eddies in the surrounding Sargasso Sea (Hela et al.,
1953). Due to shallow depths, the North
Lagoon warms and cools over a larger range; winter lows may reach
14°C, climbing to 31°C in the summer. There appear to have been
record-setting highs in the past six years (Table 1;
Cook et
al., 1990,1993), but
this may be the result of more frequent sampling and detection of
short-term peaks.

The lagoonal waters are exchanged over the
outer reef zones by semi-diurnal tides with a mean range of 0.75
m and a spring range of 1.0 m. Salinity varies little from the
36.5 ppt recorded in the North Lagoon and the outer reef zones.
The more enclosed inshore basins do show modest departures (~
35.8 ppt) due to groundwater seepage, run-off, and rainfall.

The lagoonal and inshore basins are more turbid
than the surrounding oceanic waters, due to particle resuspension
and higher water column productivity (Bodungen et al., 1982;Jickells et al.,
1986;Morris
et al., 1977). Extinction
coefficients are about 0.05 for the oceanic waters and range
between 0.13 and 0.57 on the Reef Platform. Recent Secchi data
show clear seasonal patterns at both the lagoonal seagrass site
and the outer reef site, with generally clearer conditions in
late spring/early summer, followed by a decline in water clarity
through summer and fall (Table
1).

A water quality monitoring program (BIWI) has
collected monthly surface water samples from the inshore waters
and in the North Lagoon since 1977 (Bodungen et al., 1982;Jickells et al.,
1986;Morris
et al., 1977). The program
measures dissolved nutrients (NO2+NO3, NH4, PO4), pigment concentrations, temperature, and Secchi disc
depths. Significant nitrogen enrichment was found in the inshore
basins but not in the North Lagoon. One site (Site 3A) is located
about 5 km east of the North Seagrass Site.

Climatic data are collected at two locations. Rainfall data are
collected on St. Davids Island as part of the AEROCE
program. Air temperature data are collected at the Naval
Oceanographic Command Facility, U.S. Naval Air Station, also on
St. Davids Island (see Fig. 1). oceanographic data
are collected at the Hog Breaker Reef Site, the North Seagrass
Site, and the Hungry Bay Mangrove Site.

Mangrove
Ecosystems

Mangrove communities in Bermuda are the
northernmost in the Atlantic and are limited in diversity and
development (Thomas and Logan, 1992;Thomas, 1993).
Bermudas mangrove swamps have been reduced dramatically due
to foreshore development, particularly in the last century (Sterrer and
Wingate, 1981; Thomas and
Logan, 1992). Approximately 16 ha of
mangroves remain, perhaps less than half the pre-Colonial amount,
and ~6 ha of the total are found associated with landlocked
anachialine ponds (Thomas and Logan, 1992).

Mangrove swamp development was never very
extensive due to the steeply sloped shoreline and lack of
estuarine environments. Most of the current mangroves are
classified as fringing communities and are composed of only two
species, Rhizophora mangale and Avicennia germinans,
that exist as narrow stands along the shore (Thomas and
Logan, 1992). The distribution of the
mangroves on Bermuda is disjunct, due to the character of the
coastline. Most of the mangrove communities are small (< 1 ha)
and there are no zonation patterns. Hungry Bay is the largest
swamp (2.9 ha), with a creek system that has been channelized
since the 1950s (Fig. 2). The seaward margin of the Hungry Bay swamp has
retreated significantly due to sea-level rise over the past 100
years (Ellison,
1993).

Fig. 2. Map of the mangrove forest at
Hungry Bay (after J. Ellison, 1993). The locations
of the four study plots are shown, as well as the
principal channels within the forest.

The CARICOMP mangrove study is carried out in
Hungry Bay (32°17.3'N; 64°45.5'W) on the southern shore of the
island. Four 10 x 10 m vegetation study plots are located along
the central axis of the swamp (Fig. 2) and the water sampling
station is located near the mouth of the main drainage canal.
CARICOMP results from 1992-93 indicate that salinity in the creek
may drop to 33 ppm. The mangrove trees at the seaward margin are
under stress, with reduced diameters, heights, and litter
production compared to the plots in the interior of the swamp
(CARICOMP data). Also, no seedlings are found at the seaward
margin though they are abundant within the swamp. Ellison (1993) has demonstrated that the margin of the swamp is
below mean sea level due to erosion and rising sea level, and
this has contributed to the extensive retreat of the margin over
the past 90 years.

Seagrass beds are distributed throughout the
Reef Platform but they are generally limited in extent and have
not been well studied (Thomas and Logan, 1992). The dominant species are Thalassia testudinum
and Syringodium filiforme, which can form large
monospecific stands. The most extensive Thalassia beds are
located off the southwestern end of the island. A large bed of Syringodium
is located along the North Shore (Knap et al., 1991;Smith et
al., 1995). In the inshore basins, Thalassia
and Syringodium usually occur in mixed stands along with a
third species, Halodule bermudensis. These beds are
generally small due to the limited shallow water areas
surrounding the relatively deep inshore basins. Several small
bays and sheltered coasts have been mapped (Morris et al.,
1977; Thomas and Logan, 1992). Seagrass beds are found associated with the patch
reefs in the North Lagoon, either contained within the cellular
and mini-atoll reefs (Logan, 1988)
or on their flanks. Their distribution within the North Lagoon
reef complex is patchy and has not been described. T.
testudinum and S. filiforme are the most abundant
species in these beds, occurring as either mixed beds or
monospecific stands.

The two CARICOMP seagrass sites are located within a mini-atoll
reef (Crescent Reef) in the center of the North Lagoon, south of
the north shipping channel, in an area known as The Crescent (Fig. 1
and Fig. 3). Water depths off the reef reach 15-18 m. The North
Seagrass Site (32°24'04"N; 64°47' 57"W) consists of a
broad belt of T. testudinum adjacent to the eastern reef,
which grades into an extensive mixed stand of S. filiforme
and H. bermudensis. The South Seagrass Site
(32°24'10"N; 64°48'20"W) is a monospecific stand of T.
testudinum. The seagrass beds are surrounded by banks of reef
that may shallow to 0.5 m depth but average 2-3 m in depth. Water
depth at both seagrass beds is about 5 m. Shoot densities of T.
testudinum are low (790 ± 190 [standard error] m2) and blade
lengths short (5-11 cm), perhaps indicative of the relatively
exposed condition of these beds. These sites appear to be
pristine and undisturbed by human activities, apart from the
CARICOMP study.

Fig. 3. Detailed maps of
the two reef monitoring sites and the two seagrass
study sites.

Thalassia beds are found at the lagoonward edge of the outer reef
zone of the North Lagoon, known as the Rim Reef (see Fig. 3
and Fig. 1). These beds are small (<0.5 ha), patchy in
distribution, and occur in relatively deep water (8-10 m). They
are an important habitat for protected populations of the Queen
Conch, Strombus gigas (Berg et al., 1992).

Extensive reef zones are developed at the
margin and on the shallow flanks of the Bermuda Pedestal (Fig 1; Garret and
Scoffin, 1977;Logan, 1988).
The Rim Reef is a shallow (3-10 m) zone, about 0.5-1 km wide,
that separates the ocean from the North Lagoon. This zone is
reduced on the narrow southeastern edge of the pedestal adjacent
to the islands, where a nearly continuous linear sequence of
emergent algal-vermetid reefs ("boilers") separate the
nearshore reef zone from the ocean (Meischner and Meischner, 1977). Seaward of the northern Rim Reef and southern boiler
reefs is the extensive Main Terrace, sloping between 15-30 m,
that surrounds the entire pedestal and may be up to 3 km in
width. Below the Main Terrace reefs is the deep fore-reef that
slopes sharply off and terminates at about 60 m (Fricke and
Meischner, 1985). Within the North
Lagoon is an extensive array of patch reefs varying in size and
configuration (Garret et al., 1971;Logan, 1988),
interspersed with deep (15-18 m) muddy basins.

The Rim and Terrace Reefs have similar coral
assemblages, made up of a few species, but coverage ranges from
25% in the former zone to 50% in the latter zone (Dodge et al.,
1982; Logan, 1988). Diploria
strigosa, D. labyrinthiformis, Montastraea franksisensu (Weil and Knowlton, 1994), M.
cavernosa, and Porites astreoides are the dominant
corals in these zones, along with the hydrozoan Millepora
alcicornis. Less common species include Stephanocoenia
michelini, Favia fragum, Agaricia fragilis, Madracis
decactis, Siderastrea spp., Scolymiacubensis,
and Isophyllia sinuosa. The deep fore-reef community is
composed of Montastraea spp., A. fragilis, S.
michelini, and Madracis spp. (Fricke and Meischner, 1985).

The lagoonal patch reefs have a similar Diploria-Montastraea-Porites
community structure with lesser coverage (<20%) (Dodge et al.,
1982; Garret et al., 1971). However, the patch reefs closer to the island and
within Castle Harbour support a different community of primarily
branched species (Madracis decactis, M. mirabilis, Oculina
diffusa) that grow on the vertical sides of the reefs (Dryer and Logan,
1978; Logan, 1988).
This reef community may have developed as the result of higher
sedimentation rates close to shore and the degree of protection
from wave energy. Coral diversity is greatest on these reefs,
along with other sessile invertebrates and benthic algae.

Coral growth rates appear to be seasonal, with
faster growth in the summer months but reduced for some species
compared to Caribbean congeners (Table 1; Logan and
Tomascik, 1991). Higher growth rates
for several coral species are found within the lagoonal reefs
compared to the outer reef zones (Logan et al., 1994).

Overall, Bermudas reefs are in good health, despite
repeated coral bleaching episodes (Cook et al., 1993). The reefs have suffered direct human impact (ship
groundings, dredging) but these have generally been limited in
extent (Cook
et al., 1993). The potential
effects of the over-harvesting of reef fishes have been mitigated
by a new management plan that eliminates the use of non-selective
traps and creates no-fishing zones (Butler et al., 1993). Recent monitoring has noted the recovery of some fish
stocks (Luckhurst, 1994).

The CARICOMP reef sites are located on the northern rim reef about
12 km from the island (Fig.
1 and Fig. 3). The Hog Breaker site
(32°27'32"N; 64°49'54"W) and the Twin Reefs site
(32°27'51"N; 64°48' 56"W) have been used for
extensive investigation of coral recruitment, mortality, algal
abundance, and fish grazing activity since 1986 (Smith 1988,1990,1992; Hog Breaker = Smiths WC and Twin Reefs =
Smiths EC). Monitoring of coral bleaching on permanent
transects at both sites has been carried out since 1990 (Cook et al.,
1993). Neither site shows any evidence
of human interference apart from low-impact scientific endeavors
(photography, algal collection, installation of marking stakes).

The reef sites are near the center of the Rim
Reef zone, about 100-150 m shoreward from the transition of the
Rim Reef to the Main Terrace; thus, these sites are exposed to
oceanic swell and storm waves. Both sites are characterized as a
bank of reef, average depth 7-9 m, interspersed with
sediment-filled depressions at about 10 m depth. The reef surface
is a fairly rugose relief of 1-2 m, due in part to the large
sizes (0.5-1.5 m diameter) of the main framework-builder, Diploria
spp. (Smith
1988). An unusual feature is the
presence of occasional biogenic carbonate pillars up to 4 m in
height and 1-2 m in width, with sparse coral cover (Logan, 1988).

The coral communities at both sites are the Diploria-Montastraea-Porites
assemblage typical of the Rim Reef. Gorgonian corals are common,
primarily Pseudoplexaura spp., Plexaura spp., Eunicea
spp., Pseudopterogorgia spp., and Gorgoniaventalina.
Other common sessile invertebrates are coralliomorpharians,
zoanthids, and anemones. Large erect sponges are rare. Reef turf
algae are primarily Polysiphonia spp., Ceramium
spp., Herposiphonia secunda, and Sphacelaria sp.
The most common macroalgae are Laurencia obtusa and Dictyota
bartayresii, although Ceramium nitens becomes
seasonally dominant in the summer months (Smith, 1990).

Interaction of CARICOMP Sites and
Relationship to the Caribbean Sites

The three CARICOMP study sites on Bermuda are
separated physically and do not have any direct interactions.
Bermudas study sites are geographically unrelated to the
Caribbean, with different climatic and oceanographic regimes.
However, Bermudas role as an outlier ecological system may
be a valuable point of reference for future changes that may
occur in the Caribbean.

Acknowledgements

The Bermuda CARICOMP program is supported by a
grant from the MacArthur Foundation and the Marine and
Atmospheric Programme at the Bermuda Biological Station for
Research, which is partially supported by grants from the Bermuda
Ministry of the Environment. I would like to thank
Dr. Anthony Knap, Director of BBSR, for his support of the
project. Thanks also to J. Ellison, the Earth Watch Challenge
Award Scholars (L. Rymarquis, S. Brown, S. Rosen, S. Morby, J.
Last, T. Klein), R. Morin, V. Mattin, D. Marsh, A. Holland,
T. Murdoch, S. Keyes, T. Warren, D. Hayward, C. Bosch de Noya, I.
Kuffner, D. Hellin, G. Levi, S. McKenna, J. Bell, B. White, H.
Litzenberger, and F. Minors for assisting with the establishment
and monitoring of the CARICOMP sites. Thad Murdoch prepared the
illustrations. Contribution Number 1466 from the Bermuda
Biological Station for Research, Inc.

Smith, S. R., T. Murdoch, T. Warren, D.
Hayward. 1995. Final Report of the
Benthic Monitoring Programme Prior to the Operation of the
Tynes Bay Incinerator. Report to the Ministry of Works
and Engineering, Government of Bermuda, 106 pp.

South, G. R. 1983.
A note on two communities of seagrasses and rhizophytic algae
in Bermuda. Botanica Marina, 26:243-248.